It is when one wipe their ass for any number of reason...
after they go to the bathroom and have completed their normal routine in the bathroom. they go on about their day. only to return to the bathroom later on to wipe because they don't feel clean or they just wanted to.
Sometimes you need a courtesy wipe when you get swamp ass to get rid of the sweaty butt crack.
A courtesy wipe can be needed when one has a nasty fart, this is to check for a possible shart.
One can also wipe their ass any time during the day and this can be consider a courtesy wipe.

a persons inter thoughts: {Oh man, that is not feeling right better give myself a courtesy wipe.}

I am sweating like crazy i need a courtesy wipe.

Friend A: Oh dude you shit your pants at least give yourself a courtesy wipe cause that that fart sound way to gross. it had to of been a shart.
Friend B: it was not that bad, i guess i will it is a courtesy to do so.

I need a good wiping oh yes a courtesy wipe oh much better feeling nice and clean.

When you are done wiping your ass after taking a shit and you're not sure if you wiped everything away, the decision for a courtesy wipe has to be made. This is done as a courtesy to other people that might have to deal with your stank ass from not wiping enough.

Adam: "What is that smell?"
Mike: "I don't know. I don't smell anything."
Adam: "Did you just take a shit?"
Mike: "Yes I did."
Adam: "Did you consider a courtesy wipe?"
Mike: "What is that?"
Adam: "At least we know what that smell is now"